In recent years, semiconductor devices have become more integrated, and structures of semiconductor elements have become more complicated. Further, the number of layers in multilayer interconnections used for a logical system has been increased. Accordingly, irregularities on a surface of a semiconductor device are increased, so that step heights on the surface of the semiconductor device tend to be large. This is because, in a manufacturing process of a semiconductor device, a thin film is formed on a semiconductor device, then micromachining processes, such as patterning or forming holes, are performed on the semiconductor device, and these processes are repeated to form subsequent thin films on the semiconductor device.
When the number of irregularities is increased on a surface of a semiconductor device, the following problems arise. When a thin film is formed on a semiconductor device, the thickness of the film formed at portions having a step becomes relatively small. Further, an open circuit may be caused by disconnection, or a short circuit may be caused by insufficient insulation between interconnection layers. As a result, good products cannot be obtained, and the yield tends to be lowered. Further, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after a long-term use. At the time of exposure in a lithography process, if the irradiation surface has irregularities, then a lens unit in an exposure system is locally unfocused. Therefore, if the irregularities on the surface of the semiconductor device are increased, then it becomes problematically difficult to form a fine pattern itself on the semiconductor device.
Further, as semiconductor devices have become more highly integrated in recent years, circuit interconnections have become finer and distances between those circuit interconnections have become smaller. In the case of photolithography, which can form interconnections that are at most 0.5 μm wide, it is required that surfaces on which pattern images are to be focused by a stepper should be as flat as possible because the depth of focus of an optical system is relatively small.
Thus, in a manufacturing process of a semiconductor device, it increasingly becomes important to planarize a surface of the semiconductor device. One of the most important planarizing technologies is chemical mechanical polishing (CMP). Thus, there has been employed a chemical mechanical polishing apparatus for planarizing a surface of a semiconductor wafer. In a chemical mechanical polishing apparatus, while a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such as a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
This type of polishing apparatus includes a polishing table having a polishing surface formed by a polishing pad, and a substrate holding device, which is referred to as a top ring (substrate holding device), for holding a substrate such as a semiconductor wafer. When a semiconductor wafer is polished with such a polishing apparatus, the semiconductor wafer is held and pressed against the polishing table under a predetermined pressure by the top ring. At that time, the polishing table and the top ring are moved relative to each other to bring the semiconductor wafer into sliding contact with the polishing surface, so that the surface of the semiconductor wafer is polished to a flat mirror finish.
In such a polishing apparatus, the polishing pad is so elastic that pressing forces applied to a peripheral edge portion of the semiconductor wafer tend to be non-uniform. Accordingly, the semiconductor wafer may excessively be polished at the peripheral edge portion to thus cause edge rounding. In order to prevent such edge rounding, there has been employed a top ring having a retainer ring for holding a side edge portion of a semiconductor wafer and pressing a polishing surface located outside of a peripheral edge portion of the semiconductor wafer.
Further, when a polishing apparatus employs a polishing pad made of resin, the polishing pad is worn out by dressing and polishing. In this case, in order to prevent surface pressure distribution from varying on a surface of a semiconductor wafer held by a top ring, a constant distance should be maintained between a surface of the top ring to hold the semiconductor wafer and the polishing pad during polishing. When a retainer ring, which holds a peripheral edge portion of a semiconductor wafer, is provided, the retainer ring may be worn out according to progress of polishing. When the retainer ring is thus worn out, a constant distance should also be maintained between a surface of the top ring to hold the semiconductor wafer and the polishing pad during polishing.
In order to determine whether a polishing process is performed normally in the aforementioned polishing apparatus, it is necessary to monitor a pressing force to press a semiconductor wafer, and concentration and flow rate of a polishing liquid. However, for example, various devices such as a component analyzer and a particle size distribution measuring device are required to monitor a polishing liquid. Accordingly, cost of the polishing apparatus is increased. Further, a polishing profile may also be changed by wear of the polishing pad and the retainer ring. Thus, monitoring only a pressing force and a polishing liquid is insufficient to guarantee that a polishing process is normally performed.
Further, a conventional retainer ring is configured to press a polishing surface uniformly along its overall length in a circumferential direction of the retainer ring. However, as described above, since a polishing pad used to provide a polishing surface is elastic, the polishing pad is elastically deformed so as to produce extremely increased resistance at an outermost portion of the retainer ring which is located upstream along a direction of rotation of the polishing table. Accordingly, the retainer ring is pressed downstream along the direction of rotation of the polishing table so as to cause inclination of the retainer ring. In a conventional polishing apparatus, when the retainer ring is thus inclined, a pressure under which the retainer ring presses the polishing surface is increased to prevent the semiconductor wafer from being separated from the top ring. Further, non-uniformity of the polishing profile which is caused by the inclination of the retainer ring is improved with equalization by rotation of the semiconductor wafer.
However, the conventional retainer ring has difficulty in enhancing the controllability of the temperature of the polishing pad and the polishing profile. Accordingly, in order to further enhance the controllability of the temperature of the polishing pad and the polishing profile, it is required to control a pressure under which the retainer ring presses the polishing surface along a circumferential direction of the retainer ring.